1. Field of the Invention
The present invention relates to a method for producing a Group III nitride semiconductor by the flux method. More particularly, the invention relates to a method of separating a Group III nitride semiconductor layer from a sapphire substrate while cracking of the Group III nitride semiconductor layer grown by the flux method is prevented, in the case where a template substrate formed of the sapphire substrate and the Group III nitride semiconductor layer is employed as a seed crystal.
2. Background Art
The Na flux method is a known method for producing a Group III nitride semiconductor such as GaN. In this method, a molten mixture of Na (sodium) and Ga (gallium) is reacted with nitrogen at about 800° C. and a pressure of some ten atmospheres, to thereby grow GaN crystals.
The Na flux method employs, as a seed crystal, for example, a template substrate formed of a sapphire substrate and a GaN layer grown thereon by HVPE, or a GaN substrate.
Japanese Patent Application Laid-Open (kokai) No. 2006-169024 discloses a method for growing a Group III nitride crystal, which method includes employing a GaN substrate as a seed crystal; placing the seed crystal in a molten mixture of Na and Ga at such a temperature and a pressure that crystal growth of GaN is inhibited, to thereby melt back a part of the surface of the GaN substrate and form grooves; and subsequently, growing a GaN crystal on the GaN substrate at such a temperature and a pressure that crystal growth of GaN is allowed, to thereby fill the grooves with the molten mixture. In this method, since the grooves are filled with the molten mixture, the heat radiation performance of the GaN crystal is effectively improved.
Japanese Patent Application Laid-Open (kokai) No. 2004-247711 discloses a method for producing a Group III nitride free-substrate, which method includes employing, as a seed crystal, a template substrate formed of a sapphire substrate and a stripe-pattern GaN layer formed thereon; growing GaN on the a stripe-pattern GaN layer through the Na flux method; and subsequently, cooling the stacked product, to thereby generate stress attributable to the difference in linear expansion coefficient between sapphire and GaN and separate the seed crystal, whereby a high-quality GaN free-standing substrate is produced. The patent document also discloses that the stripe-pattern GaN layer may be formed through dry-etching a part of the GaN layer to such a depth that the surface of the sapphire substrate is exposed.
However, the second method is very cumbersome, since the method includes the dry-etching step of forming the stripe-pattern GaN layer and the step of growing GaN crystal by the Na flux method, separately.